Shear thickening fluids (STFs) are fluids whose viscosity increases with shear rate. Of particular interest are discontinuous STFs, which at high shear rates transform into a material with solid-like properties. A typical example of a discontinuous STF is a stabilized suspension of rigid colloidal particles with a high loading fraction of particles. Such systems have been studied for many different combinations of fluid matrix and particle size and compositions (Egres, R. G., Lee, Y. S., Kirkwood, J. E., Kirkwood, K. M., Wetzel, E. D., and Wagner, N. J., “Novel flexible body armor utilizing shear thickening fluid composites.” Proceedings of 14th International Conference on Composite Materials. San Diego, Calif. Jul. 14-18, 2003), (Lee, Y. S., Wagner, N. J., “Dynamic properties of shear thickening colloidal suspensions,” Rheol Acta 42, 199-208 (2003), (Shenoy, S., Wagner, N. J., Bender, J. W., “E-FiRST: Electric field responsive shear thickening fluids,” Rheo Acta 42, 287-294 (2003), Barnes “Shear-thickening (“dilatancy”) in suspensions of nonaggregating solid particles dispersed in Newtonian liquids”, J. Rheology 33, 329-366 (1989)). The shear thickening in the colloidal suspension is due to the formation of jamming clusters, or hydroclusters, (Lee, Y. S., Wagner, N. J., “Dynamic properties of shear thickening colloidal suspensions,” Rheol Acta 42, 199-208 (2003)) bound together by hydrodynamic lubrication forces. The hydrocluster growth and collision eventually result in a percolated arrangement of the rigid particles across macroscopic dimension. This microstructural transformation leads to the bulk solid-like behavior. Upon relaxation of the applied stresses, the rigidized material typically relaxes to the low strain rate, fluid-like behavior (Eric D. Wetzel, Y. S. Lee, R. G. Egres, K. M. Kirkwood, J. E. Kirkwood, and N. J. Wagner, “The Effect of Rheological Parameters on the Ballistic Properties of Shear Thickening Fluid (STF) KEVLAR®Composites” NUMIFORM, 2004).
Shear-thickening fluids have been shown to have utility in the fabrication of energy dissipative devices, such as shock absorbers (Hesse, H., U.S. Pat. No. 4,503,952), (Rosenberg, B. L., U.S. Pat. No. 3,833,952), (Sheshimo, K., U.S. Pat. No. 4,759,428) and more recently in the fabrication of ballistic fabric composites (Egres, R. G., Lee, Y. S., Kirkwood, J. E., Kirkwood, K. M., Wetzel, E. D., and Wagner, N. J., “Novel flexible body armor utilizing shear thickening fluid composites.” Proceedings of 14th International Conference on Composite Materials. San Diego, Calif. Jul. 14-18, 2003), (Lee, Y. S., Wetzel, E. D., and Wagner, N. J., “The ballistic impact characteristics of KEVLAR® woven fabrics impregnated with a colloidal shear thickening fluid”, J. Mat. Sci. 38, 2825-2833 (2003), (Eric D. Wetzel, Y. S. Lee, R. G. Egres, K. M. Kirkwood, J. E. Kirkwood, and N. J. Wagner, “The Effect of Rheological Parameters on the Ballistic Properties of Shear Thickening Fluid (STF) KEVLAR® Composites” NUMIFORM, 2004). There is considerable interest in incorporating STF's into other materials. PCT/US2004/015813 entitled “Advanced Body Armor using a shear thickening fluid” is incorporated by reference in its entirety for all useful purposes. Incorporation of STF's into plastics, rubbers and foams is discussed below. Shear thickening fluids may also contain fillers, see PCT application no. US06/04581 filed Feb. 9, 2006, which is incorporated by reference in its entirety for all useful purposes.
Within the scope of this invention, the shear thickening fluid is defined as any fluid that exhibits an increase in viscosity with increasing shear rate or applied stress. Shear thickening is not shear dilatancy, which is a material property whereby the material's volume changes upon an applied stress or deformation. Shear thickening fluids, however, may exhibit dilatancy under specific conditions.
Emulsions of two immiscible or partially miscible fluids have been extensively explored in several areas of research. Shear thickening “suspoemulsions” have been developed in a previous patent application (Wagner, Egres, Kirkwood, 2004 (U.S. Ser. No. 11/260,742 which is incorporated by reference in its entirety for all useful). However, the emulsification of highly concentrated dispersions of particles, such as shear thickening fluids (STFs), into volatile solvents, such as water, has not been reported previously, either in the aforementioned patent, or in the literature. Novel methods to emulsify dispersions into an immiscible or partially miscible carrier fluid are described in this patent application.
Typical processing of STF-fabric composites involves the use of copious amounts of a volatile solvent that can solubilize the STF, i.e., a co-solvent, such as ethanol (see prior art) to dilute the STF (approximate 50% by vol. silica particles dispersed in a polymeric matrix such as silicone oil). The use of ethanol not only poses potentially serious health and safety risks but also introduces process design challenges due to fire safety and VOC regulations. Further, the use of a co-solvent poses problems in that the particles can sediment out of the diluted solution, or the co-solvent may induce particle aggregation or precipitation. Despite these issues, ethanol is currently used in its present technology due to its benefits in STF-fabric processing: STF easily dissolves in ethanol and thus allows for ease in coating and manufacturing; ethanol can easily be removed to leave behind only STF in fabrics. The use of water instead of ethanol would eliminate any safety or health hazards. However, as environmentally stable STFs are formulated with water insoluble or sparingly water soluble carrier fluids, water cannot be directly used as a co-solvent to dilute the STF. Hence, the challenge is to develop a method whereby a STF can be emulsified as a dispersed phase in an aqueous solution. An emulsion refers to a state of matter whereby a fluid phase, which may contain multiple components including particles, polymer, and or surfactants, is dispersed as droplets in an insoluble or sparingly soluble fluid. Further, the subsequent challenge is to maintain the stability of the emulsion as well as the integrity of the STF phase upon drying or separation of the aqueous carrier fluid. Neither of these specific challenges has been addressed in the literature.